The regional distribution of ventilation and blood flow in the human lung will be determined using the radioactive xenon method. The efficacy of ventilation/perfusion matching as an adaptive mechanism in hypoxia will be assessed by observing the effect of alterations of the inspired oxygen tension on normal subjects, high altitude residents and patients with hypoxic pulmonary disease. Patients with valvular and ischemic heart disease will be studied at rest, during exercise and after the administration of drugs to gain information about the behavior of the pulmonary circulation in cardiac disease by a non-invasive method. Simultaneous measurements of regional lung function and overall lung mechanics will be made to assess the factors affecting the local distribution of gas and blood flow in the lungs. These studies will give further information on the factors controlling small bronchi and blood vessels in the lungs and will assist the interpretation of clinical lung scans.